The use of wireless networks has increased dramatically. WLANs are now commonplace in the modern workplace and becoming more prevalent in many universities and households in the United States. A WLAN offers several advantages over regular local area networks (“LANs”). For example, users are not confined to specified locations previously wired for network access, wireless work stations are relatively easy to link with an existing LAN without the expense of additional cabling or technical support; and WLANs provide excellent alternatives for mobile or temporary working environments.
There are generally two types of WLANs, independent and infrastructure WLANs. The independent, or peer-to-peer, WLAN is the simplest configuration and connects a set of personal computers with wireless adapters. Any time two or more wireless adapters are within range of each other, they can set up an independent network. In infrastructure WLANs, multiple access points link the WLAN to the wired network and allow users to efficiently share network resources. The access points not only provide communication with the wired network, but also mediate wireless network traffic in the immediate neighborhood. Both of these network types are discussed extensively in the IEEE 802.11 standard for WLANs.
In the majority of applications, WLANs are of the infrastructure type. That is, the WLAN typically includes a number of fixed access points, also known as base stations, interconnected by a cable medium to form a hardwired network. The hardwired network is often referred to as a system backbone and may include many distinct types of nodes, such as, host computers, mass storage media, and communications ports. Also included in the typical WLAN are intermediate base stations which are not directly connected to the hardwired network.
These intermediate access points, often referred to as wireless base stations, increase the area within which access points connected to the hardwired network can communicate with mobile terminals. Associated with each access point or base station is a geographical cell. A cell is a geographic area in which an access point has sufficient signal strength to transmit data to and receive data from a mobile terminal with an acceptable error rate. Unless otherwise indicated, the term access point or base station, will hereinafter refer to access points hardwired to the network and wireless base stations. Typically, the access point connects to the wired network from a fixed location using standard Ethernet cable, although in some cases the access point may function as a repeater and have no direct link to the cable medium. Minimally, the access point receives, buffers, and transmits data between the WLAN and the wired network infrastructure. A single access point can support a small group of users and can function within a predetermined range.
In general, end users access the WLAN through WLAN adapters, which are commonly implemented as Personal Computer Memory Card International Association or Peripheral Component Microchannel Interconnect Architecture, commonly referenced as “PCMCIA” cards in notebook computers, Industry Standard Architecture (“ISA”) or Peripheral Component Interface (“PCI”) cards in desktop computers, or fully integrated devices within hand-held computers. WLAN adapters provide an interface between the client network operating system and the airwaves. Typically, the nature of the wireless connection is transparent to the network operating system.
In general operation, when a mobile terminal or a portable unit is powered up, it “associates” with an access point through which the mobile terminal can maintain wireless communication with the network. In order to associate, the mobile terminal must be within the cell range of the access point and the access point must likewise be situated within the effective range of the mobile terminal. Upon association, the mobile unit is effectively linked to the entire LAN via the access point. As the location of the mobile terminal changes, the access point with which the mobile terminal was originally associated may fall outside the range of the mobile terminal. Therefore, the mobile terminal may “de-associate” with the access point it was originally associated to and associate with another access point which is within its communication range.
Due to the multiple sources and types of communications which occur over a WLAN, signals are encoded over multiple channels using complex modulation techniques, such as orthogonal frequency division multiplexing (“OFDM”) whereby the datastream is split into multiple RF channels. Note, the invention described below, can be applied to any wireless medium that utilizes complex encoding of the packet or data frame, including the IEEE 802.11g standard. Due to the complex encoding schemes utilized in WLAN's and the variable packet size discussed below, complex calculations must occur in a very short amount of time to maintain compliance with various WLAN standards such as 802.11 and to maintain a high system throughput.
In meeting the responsibilities set forth above, a significant portion of the MAC processing time is spent determining the proper time to send an acknowledgement (“ACK”) signal. In 802.11a, the MAC must send an acknowledgement (“ACK”) signal within a short inter frame space (“SIFS”) of 16 μs from receiving the end of a packet.
One problem associated with the prior method is that the MAC does not have any knowledge of the received signal envelope in the air, since the symbol decoding process takes some variable amount of time, the MAC processor typically has to respond to the incoming packet in a very short amount of time with accurate timing, as stated above, within a SIFS. Currently, there is a fixed timing reference point in the packet to determine timing. In many cases, this fixed timing reference will be generated before any packet data is transferred. A timing reference may be difficult to detect while the MAC processor is busy receiving packet data. However, given this timing reference at the beginning of the packet, it is still quite difficult to determine the end of the packet or data frame. The total time of the packet must be calculated using a complex algorithm, and an adjustment must be applied based on the knowledge of the relationship between the beginning of the packet and the timing reference signal. Given the decoding delay of the received data, this calculation must be performed in a very short amount of time.
As shown in FIG. 1, the leading edge of the Timing Reference signal has a fixed relationship to the RF Packet, but the trailing edge of the Timing Ref signal is variable (shown in FIG. 1 as the crossed section 112 of the Timing Ref signal). This is typical because of the varying decoder delays depending on the length of the packet and the data rate. In order to transmit the ACK with the correct 16 μs timing at the end of the packet, the total length of the packet must be calculated, then the time “A” (shown in FIG. 1) must be subtracted from it, and the 16 μs delay must be added to it. As will be understood by those skilled in the art, the time “A” corresponds to the time from the beginning of the receipt of the RF Packet to the beginning of the Timing Ref signal. The length calculation for 802.11a isTXTIME=TPREAMBLE+TSIGNAL+TSYM*Ceiling((16+8*LENGTH+6)/NDBPS)Where the TXTIME is the entire time for the transmitting the sum of the following variables: the packet preamble (TPREAMBLE), the time to transmit the signal field (TSIGNAL), and the time to transmit the data portion of the packet (TSYM*Ceiling((16+8*LENGTH+6)/NDBPS)), where TSYM is the symbol time in microseconds, LENGTH is the packet length in bytes, and NDBPS is the number of bits per symbol. This is an extremely complex calculation to do in real time and will take a significant amount of processor cycles to accomplish. This is not a desirable solution because of the computational complexity involved and the number of MAC processor cycles required to attend to basic synchronization functions.
Thus, there exists a need for a system and method that can eliminate or significantly reduce the complexity of the above calculation or thereby free the MAC processor from performing this synchronization task and thereby increase throughput of the WLAN system and eliminate timing errors.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of instrumentalities and combinations particularly pointed out in the appended claims.